1. Field of the Invention
The present invention relates to an X-ray image intensifier, particularly, to an improvement in the input screen of the X-ray image intensifier.
2. Description of the Related Art
FIG. 1A shows the input screen of a conventional X-ray image intensifier. As seen from the drawing, the input screen comprises input substrate 31 having a smooth surface, a first phosphor layer consisting of CsI:Na crystal grains formed on input substrate 31 by vapor deposition under a low degree of vacuum, second phosphor layer 34 consisting of CsI:Na crystal grains grown in a columnar shape on the first phosphor layer, surface layer 35 consisting of CsI:Na phosphor formed on the second phosphor layer 34 by vacuum deposition under a high degree of vacuum, and a photocathode 36.
Second phosphor layer 34 consists of columnar CsI crystals grown in a direction substantially perpendicular to the surface of input substrate 31. Columnar crystals have an average diameter of 5 to 50 microns and a length of about 400 microns. The columnar crystals are separated from each other by fine clearance 33. When photocathode 36 is formed directly on the surface of the second phosphor layer 34 consisting of the columnar crystals, photocathode 36 is also divided into fine island-shaped regions. In photocathode 36 of this shape, an electric connection cannot be achieved in a direction parallel to the surface of photocathode 36. It follows that it is impossible to maintain constant the potential of photocathode 36 with increase in the number of photoelectrons emitted from photocathode 36. As a result, the electrooptic uniformity of the X-ray image intensifier is markedly impaired, leading to distortion of the output image or reduction of resolution.
To overcome the difficulty, surface layer 35 is formed on second phosphor layer 34, followed by forming photocathode 36 on surface layer 35. Since surface layer 35 has a relatively continuous surface, photocathode 36 formed on surface layer 35 also has a relatively continuous surface, with the result that it is possible to ensure an electric connection in a direction parallel to the surface of photocathode 36.
However, clearances 33 formed between the individual columnar crystals in second phosphor layer 34 include relatively large clearances 34, sized about 1 micron, which are distributed over the entire region of second phosphor layer 34, as shown in FIG. 1B. As a result, pin holes 37 corresponding to relatively large clearances 33 are formed in surface layer 35. These pin holes 37 give a detrimental effect to the sensitivity of photocathode 36. Specifically, the material of photo-cathode 36 is gradually diffused through pin holes 37 into the phosphor layer in the step of forming photocathode 36 which is carried out at such a high temperature as 100.degree. C. or more, leading to a low sensitivity of the photocathode formed. The diffusion also takes place even after completion of the step for forming photocathode 36. Accordingly, the sensitivity of the photocathode is gradually lowered, leading to a shortened life of the input sereen.
It is possible to diminish pin holes 37 and to decrease the number of pin holes 37 by increasing the thickness of surface layer 35. As a result, the sensitivity of photocathode 36 can be improved. However, the increased thickness of surface layer 35 brings about a low resolution of the input screen, leading to a low resolution of the X-ray image intensifier. Under the circumstances, the thickness of surface layer 35 is practically set at about 10 to 30 microns.
It should also be noted that photocathode 36 itself has a high electric resistance in some cases depending on the materials of photocathode 36, making it impossible to put the input screen into practical use even if photocathode 36 is formed on surface layer 35 having a relatively continuous surface. In this case, a conductive intermediate layer is formed between surface layer 35 and photocathode 36. The conductive intermediate layer should desirably be highly transparent. An indium oxide film or an indium tin oxide film is known as a desirable material of the conductive intermediate layer. Even in the case of using such a conductive film, however, it is necessary to set the thickness of the intermediate layer at 0.3 micron or less in order to obtain a high enough transmittance in (.gtorsim.70%) CsI phosphor layer activated by Na. It follows that the use of a conductive intermediate layer is quite incapable of eliminating the pin holes present in the surface layer. Also, it is quite impossible to solve the problem even if the surface layer is formed by vapor deposition of a transparent material other than the phosphor.
Japanese Patent Disclosure No. 63-88732 teaches the idea of shaving the surface region of a first CsI phosphor film consisting of completely dispersed phosphor particles, followed by forming a second CsI phosphor layer by vapor deposition on the shaved surface of the first CsI phosphor film so as to provide a continuous phosphor layer surface. However, it is difficult to prevent the pin hole occurrence by the technique of this prior art.
As described above, the phosphor layer surface in the input screen of a conventional X-ray image intensifier is not sufficient continuous, but contains a large number of pin holes. The presence of the pin holes makes it difficult to form a photocathode having a high sensitivity and a long life.